MAX797H
_______________Detailed Description
The MAX797H is functionally identical to the MAX797.
The only difference between the two devices is in the
BST pin’s absolute maximum rating. The MAX797H’s
rating is 46V; the MAX797’s rating is 36V. The higher
rating allows the MAX797H to use a power input up to
40V, provided that the V+ pin is powered by a separate
supply between 4.5V and 30V.
Circuit design and component selection for the
MAX797H are identical to those for the MAX797; there -
fore, such information is not included in this data sheet.
Refer to the MAX796/MAX797/MAX799 data sheet for
design formulas and applications information. The
Applications Information
section in this data sheet con -
tains suggestions for providing the 30V maximum V+
supply input for the MAX797H when power input
exceeds 30V.
__________Applications Information
Powering the V+ Pin
V+ can be supplied directly if a system supply between
4.5V and 30V is available (see the
Typical Operating
Circuit
). Most of the MAX797H’s internal blocks are sup -
plied by VL, which uses V+ as its input. While the cur -
rent into V+ is minimal, it depends heavily on the type of
external MOSFET used and the switching frequency:
I
GATE
= Q
g
x f
SW
where Q
g
is the sum of the high- and low-side
MOSFET’s total gate charges, and f
SW
is the switching
frequency. Furthermore, if the circuit output voltage on
CSL exceeds the VL/CSL switchover voltage, the
MAX797H bootstraps itself (it connects VL to CSL and
turns off the linear regulator, supplying the IC from the
circuit output), and V+ current is reduced to about 1µA.
If a 5V regulated supply is available, V+ and VL can be
connected and fed from that supply (Figure 1). In this
mode, the VL regulator is bypassed. Do not use this
approach if the output voltage on CSL can exceed the
VL/CSL switchover voltage.
If a 5V regulated supply is not available, a linear regula -
tor with a sufficient input voltage range can provide it
(Figure 2). This approach allows for a very wide input
voltage range, which is useful if the circuit must run from
several different power sources. The drawback of the
linear regulator is the high quiescent current that these
devices typically require, in addition to the current used
by the feedback divider resistors (R1 and R2).
For most applications, a better choice than Figure 2’s
circuit takes advantage of the MAX797H’s internal lin -
ear regulator. There is no need to provide a regulated
supply to V+, provided it is within the +4.5V to +30V V+
input voltage range. In Figure 3, Q1 is used to drop a
40V (max) input to 30V by dividing it by approximately
4/3. This approach results in a somewhat higher mini -
mum input voltage than that of Figure 2’s circuit, but a
much lower quiescent current than that of a linear regu -
lator. If quiescent current must be minimized, an
N-channel MOSFET can be substituted for Q1, and the
divider-resistor values can be increased.
Powering V+ with a zener diode can be done in many
different ways. The simplest is to use a standard shunt
regulator to provide a regulated voltage in the 4.5V to
30V range (Figure 4). Resistor R1 must be chosen to
allow the maximum required V+ current to be obtained
from the minimum power input voltage. If the power
input voltage varies appreciably, the result is higher-
than-necessary input current from the highest power
input voltage. An approach that reduces quiescent
current is to use a zener diode as a dropping diode to
keep V+ under 30V (Figure 5). This results in a severely
restricted minimum range for the power input voltage,
which is not a problem for most high-voltage applica -
tions. RL must be added to draw current and to ensure
that there is sufficient forward drop across the zener
diode if the MAX797H can be shut down or bootstrap
off its output voltage.
Duty-Factor Limitations for
Low V
OUT
/V
IN
Ratios
The MAX797H’s output voltage is adjustable down to
2.5V (min). However, the combination of high input and
low output voltages may not be possible at high switch -
ing frequencies without introducing some amount of
frequency instability. The minimum duty factor is deter -
mined by delays through the error comparator, internal
logic, gate drivers, and external MOSFETs. The delay is
typically 425ns. With a switching period of 3.33 µs
(300kHz), the minimum duty factor is 0.425 µs / 3.33 µs
= 0.13. If V
OUT
/ V
IN
is less than this value, the IC will
properly regulate the output voltage, but may extend
the period and switch at 150kHz instead of 300kHz. It
may also alternate between these two frequencies. For
example, if V
IN
is 40V, the lowest V
OUT
that does not
require less than the minimum duty factor is 40V x 0.13
= 5.2V. Below this output voltage, select the 150kHz
switching frequency (connect SYNC to VL or GND).
High-Voltage, Step-Down Controller with
Synchronous Rectifier for CPU Power
6 _______________________________________________________________________________________
